Voltage feedback tuned oscillators are well known and often used because of their several advantages. They are simple and reliable, often consisting of only a transformer, a capacitor and a pair of transistors providing current paths for each half cycle of AC output. They bypass all other frequencies but the fundamental frequency of oscillation, and, therefore, they provide good input noise suppression, good input power factor and inherent output short circuit protection.
Tuned oscillators operate on the principle that the active device permit power gain at the frequency of oscillation. Thus, the active device must have sufficient gain to overcome circuit losses and establish exactly unity gain around the feedback loop. In addition, the phase shifts introduced by the active device and the feedback network must result in exactly zero phase shift around the overall circuit.
U.S. Pat. No. 3,775,702 discloses a transformer coupled, resonant feedback, tuned oscillator power supply with provision for sensing the variation in the voltage across the secondary winding of the transformer by means of a center-tapped feedback winding which responsively adjusts the value of the feedback voltage e.sub.f to the base elements of the amplifier transistors to thereby adjust the primary current, whereby the output current which flows through the load is maintained at a constant value. That oscillator requires that the transformer core loss resistance (Rc) remain substantially constant over the operating range of the power supply in order to provide the constant current output. A transformer core with a constant core loss resistance (Rc) is not readily commercially available and requires special design which adds significantly to the cost of the disclosed power supply. Thus, there is a need for a transformer coupled, resonant feedback, tuned oscillator which can provide a constant current output at different loads with a commercially available inexpensive transformer core that does not require a constant core loss resistance (Rc). Also, if such an oscillator has a feedback control network that responds only to load current demand, tighter load current regulation can be accomplished. SUMMARY OF THE INVENTION
In accordance with the invention, load current regulation of a transformer coupled, resonant feedback, tuned oscillator circuit is achieved by incorporating into such an oscillator circuit an additional feedback circuit which provides a DC error voltage to the oscillator circuit which enables it to compensate for variations in core loss resistance, load and other circuit parameters. The oscillator circuit of the invention is comprised of an arrangement of an oscillator section and a load current control network section. The oscillator section includes a voltage feedback transformer having a center-tapped primary winding, a center-tapped feedback winding and a secondary winding. Two transistors having their respective collectors connected to opposing ends of the primary winding are the amplifier elements of the oscillator. The output voltage across the secondary winding, as sensed by the feedback winding, is connected to the base elements of transistors in order to provide necessary feedback to cause sustained oscillation. A fixed capacitor C along with distributed capacity C1 connected across the transformer primary or secondary winding and the magnetizing inductance Lm of the transformer are the parameters of the frequency determining tank circuit of the osciallator.
The load current control network section consists of two separate electronic switches both of which are connected to a voltage averaging circuit and an error amplifier which compares the voltage of the averaging circuit against an adjustable reference voltage. Total return current of the transformer output is sensed by a resistor and processed by the electronic switches such that the voltage applied to the averaging network corresponds only to the ideally rectified load current component and the capacitive leakage current component which, being a symmetrical AC waveform, will have an average value of zero. The error voltage E.sub.c of the error amplifier produced by comparing the voltage across the averaging network against an external reference input voltage is applied to the center-tap of the feedback winding through a current limiting resistor. The reference input voltage sets the error voltage E.sub.c which controls the load current.
According to the invention, the transformer primary current consists of a waveform having a sinusoidal component and a squarewave component. The amplitude of the first component is determined by the feedback winding and is proportional to the output voltage that appears across the secondary winding. This provides for the magnetizing current of the transformer and only a part of it is coupled to the load current. Proper selection in the amplitude of this waveform ensures a sustained oscillation. The second squarewave component provides for the load current. Any variation in the load current in the secondary winding of the transformer is sensed by a control network which responsively adjusts the error voltage E.sub.c applied to the base elements of the amplifier transistors via the feedback winding to thereby adjust the primary current of the transformer, whereby the output current which flows over the load is maintained at a constant value.